This is true with all passbook QR and Bar codes. You use the crown or just flick the screen up.
It also sets the screen to the brightest level to help with it being read.
Well, this is actually not true.
Just tried to board another flight.
The Fly Delta app wouldn't load. I had saved my boarding pass to Passbook. Opened it there. Scrolled to the QR code. Display shut off.
Raise wrist to turn on display. Set to open clock face. Went back to passbook. Fumbled around with people grumbling.
Pulled out my paper boarding pass.
Fail.